Despite Democrats' intensifying efforts to vilify the Koch brothers, a new poll shows most Americans still do not know who they are.

The poll by George Washington University shows 52 percent of Americans have never heard of the Koch brothers. The billionaire businessmen are major donors to conservative causes and helped fund some of the country's leading conservative advocacy groups. In advance of the 2014 midterms, Senate Majority Leader Harry Reid and his Democratic colleagues have launched persistent political attacks against them.

Still, they might be breaking through to some people. According to the GWU poll, among those who did know who the Koch brothers are, more had a negative view of them. A quarter of those surveyed had an unfavorable view of the Koch brothers, while 13 percent had a favorable view.

The poll of 1,000 registered likely voters was taken March 16-20, and had a margin of error of 3.1 percentage points.

Charles Koch, 78, and David Koch, 73, inherited a small oil company from their father. They later expanded worldwide into chemicals, textiles, paper and other products, building a hugely profitable and privately held conglomerate.

Long active in conservative politics, they helped found Americans for Prosperity, which reported spending $122 million on elections in 2012.